Photorheostat including discharge lamp and masking means and utilizing length of discharge



Dec. 3, 1968 M NICOLAS PHOTORHEOSTAT INCLUDING DISCHARGE LAMP AND MASKING MEANS AND UTILIZING LENGTH OF DISCHARGE Filed Jan. 27, 1966 G3 IFlgJL F102.

United States Patent 3,414,730 PHOTORHEOSTAT INCLUDING DISCHARGE LAMP AND MASKING MEANS AND UTI- LIZING LENGTH OF DISCHARGE Michel Nicolas, Bievres, France, assignor to Compagnie Industrielle Francaise des Tubes Electroniques, Societe Anonyme, Courbevoie, France, a French company Filed Jan. 27, 1966, Ser. No. 523,332 Claims priority, application France, Feb. 5, 1965, 4,509 14 Claims. (Cl. 250217) This invention relates to photorheostats and has specific reference to an improved photorheostat of the type adapted to be incorporated in a gain control circuit of an amplifier capable of producing a sound volume compression effect.

Photorheostats are already known which consist of a photoelectric cell disposed in front of a variable light source, the assembly being enclosed in a perfectly lighttight enclosure. Any variation in the intensity of the light source is attended by a proportional variation in the photoelectric current, that is, of the cell conductance in case this cell is photoconductive. Any light source may be used, provided that the emitted spectrum is at least partially coincident with the spectrum of the cell sensitivity; thus, as light sources suitable for carrying out this invention, incandescence or filament lamps, discharge lamps, electroluminescent compounds, etc., may be mentioned.

Now in circuits of the type mentioned hereinabove the range of variation of the photocell conductance must be as wide as possible. Therefore, the law of variation of the cell characteristics must be modified to make it more directly proportional to, while evolving much more rapidly than, that of the light source; this actually is the scope of the present invention which affords variations in the photocell characteristics that can be more than a hundred times superior to those of the light source.

To this end, various methods are applicable which are based on a suitable modification of the laws governing the transmission and/ or emission of light from the light source to the photocell.

According to a preferred form of embodiment of this invention, the light transmission may 'be rendered variable as a function of the luminous intensity of the light source by using screens, whether gradual or not, attenuators, etc.

According to another form of embodiment, the light emission may vary as a function of parameters governing its intensity; thus, for instance, the cathode of a discharge lamp may be given a suitably designed shape to this end.

Finally, the above-mentioned effects may be superposed to each other, thus obviously producing an increment in the relative variation of the conductance with respect to that of the signal supplied to the light source.

The present invention relates to a photorheostat comprisin a photoelectric cell, for example a photoconductive cell, illuminated by a discharge lamp. (In connection with such discharge lamp it is known that beyond a certain voltage threshold called ignition threshold, a current develops in the anode-to-cathode gap of the discharge lamp and a series-connected protection resistance, this current producing a so-called cathode glow forming a light sheath surrounding the cathode. When the voltage increases between the anode and cathode of said discharge lamp, the current increases through the lamp and the surface area of the light sheath increases substantially in proportion to this current until the complete surface of the cathode is covered by said light sheath.)

In a photorheostat of the aforesaid discharge-lamp type the variation in the surface area of the cathode sheathed by the cathode glow is utilized for varying the photo- "ice electric current, that is, the cell resistance in the case of a photoconductive cell. The light source luminance and correlatively the value of the photoelectric current can vary only within relatively narrow limits, the lower end of the variation range corresponding to the minimum current necessary for producing a stable lamp discharge, the upper end of this range corresponding to the maximum current producing the total sheathing of the cathode under constant luminance conditions.

According to the aforesaid preferred form of embodiment of this invention, the photorheostat constituting the subject matter thereof comprises at least one photoelectric cell disposed in front of a variable light source consisting of a discharge lamp receiving a variable voltage between its anode and cathode, and is characterized in that the discharge lamp is of the type comprising a preferably filiform cathode parallel to the longitudinal axis of said lamp and that a screen of constant or variable opacity along its length is disposed between said light source and said cell. Thus, this screen being disposed along part of the cathode length and embracing the cathode end adjacent to the anode, will prevent or at least attenuate the direct transmission of the photocell of the light flux emitted by said end portion of the cathode which is adjacent to the anode, so that when the discharge begins the photocell is illuminated by a diffuse light.

Due to the use of this intermediate screen and to the emission of a diffuse light under low discharge currents, the range of variation of the light intensity of the lamp is widened, this implying correlatively a wider range of variation in the photoelectric current.

The opaque screen is advantageously disposed between the lamp and the cell, with due consideration for the minimum and maximum lengths of the cathode light sheath. This screen may consist of any suitable material capable of arresting wholly or partially the light rays and consistent with the room temperature; thus, paint, opaque paper, plastic, etc., and, of course, metals, may be used to this end.

The photorheostat according to this invention is also advantageous in that it can be manufactured from any conventional and known component elements.

In order to afford a clearer understanding of this invention and of the manner in which the same may be carried out in practice, reference will now be made to various forms of embodiment of the photorheostat constituting the subject-matter thereof which are shown in the accompanying drawing, in which:

FIGURE 1 is a diagrammatic longitudinal section of a photorheostat according to this invention, fed with direct current;

FIGURE 2 is a diagrammatic longitudinal section of an alternate form of embodiment of this photorheostat;

FIGURE 3 is a similar longitudinal section showing a photorheostat fed with alternating current;

FIGURES 4, 5 and 6 are diagrammatic elevational views showing photoconductive cells suitable for use in the photorheostat according to this invention.

The photorheostat illustrated in FIGURE 1 comprises in a light-tight enclosure 1 a photoconductive cell 2 connected through conductors 3 and 4 to a load circuit. Registering with or in front of the photoconductive cell 2 is a neon glow or discharge lamp 5 parallel to said cell; in this example the lamp 5 is of the cylindrical type. This lamp comprises two filiform electrodes, namely an anode 6 and a cathode 7, connected through separate leads 9 and 10 to a direct-current circuit delivering a variable input voltage. The anode 6 and cathode 7 are aligned with the longitudinal axis of lamp 5. Surrounding the tubular bulb of lamp 5 is an opaque screen consisting in the example illustrated and described herein of a cylindrical sleeve.

The photorheostat operates as follows:

Below a predetermined voltage threshold between the anode 6 and cathode 7, no current flows through the lamp. When the ignition voltage value is attained current suddenly flows through the gap between the anode 6 and cathode 7. The value of this current is limited by a protection resistance (not shown) mounted in series with the lamp, the resistance value being properly selected. The passage of current through the lamp produces a glow constituting a luminous sheath surrounding the cathode; at the lowest current producing a stable discharge this glow is limited to the zone G extending to a short distance from the lower end of cathode 7 which is adjacent to the anode 6 to the point L indicated in FIGURE 1. This light sheath G is not received directly by the cell due to the presence of screen 8 but produces a diffuse light which reduces to 1M9 the resistance of the photoconductive cell 2 which in complete darkness was, say lOMQ. The length of the luminous sheath G to point L may be in this case of the order of 3 millimeters. Under these conditions the resistance of cell 2 is 1M9 with a 0.1-rna. current.

If the voltage between the anode 6 and cathode 7 is subsequently increased, the current also increases and therefore the length of the cathode light sheath increases accordingly. For a certain value of the discharge current the length of the light sheath is such that its movable end begins to directly illuminate the upper edge 2a of cell 2. In this case the extent of the light sheath is G When the light sheath changes from G to G the intensity of the diffuse light increases correlatively and causes a proportional reduction in the resistance of the cell 2.

If the current further increases until the cathode luminous sheath attains its maximum permissible length G that is, to point L at the upper end of the cathode, the resistance of cell 2 continues to decrease. In the limit case wherein the light sheath extends as far as point L this point illuminates the lower edge 2b of the photoconductive cell 2. In this case, the resistance of cell 2 is 20KQ and the discharge current is 1 ma.

It will thus be seen that the resistance ratio for the minimum range G and maximum range G of the light sheath is 1,000,000/20,0G0=50, whereas the current ratio is 1/0.1=10'.

The position of cell 2 must be such that its upper edge 2a be constantly concealed by the screen from the minimum light sheath G so that under all circumstances the initial period of illumination of cell 2 is only by diffuse light.

On the other hand the point L corresponding to the maximum sheath dimension must be aligned with the upper edge 8a of screen 8 and the lower edge 2b of cell 2.

The photorheostat described hereinabove with reference to FIGURE 1 may be so modified that a greater variation in the resistance of cell 2 corresponds to a same relative variation of the discharge current. To this end, the photorheostat may be constructed as shown in FIG- URE 2.

In this figure it will be seen that the length of the cathode 7 is increased at the expenses of that of anode 6, the latter then having no optical function in the device. This accounts for an increase in sensitivity.

The photorheostat according to this invention may also be used when the discharge lamp is energized by alternating current, as shown in FIGURE 3. In this case the ratio between the endmost values of the resistance of cell 2 is nearly twice the value indicated for a same variation in the discharge current under direct-current conditions. This is due to the fact that the two electrodes 6 and 7 of the glow lamp act by turns as a cathode and are alternatively covered by the light sheath illuminating the photoconductive cell 2 disposed symmetrically in relation to the transverse plane of symmetry xy of the discharge lamp 5. The upper edge 3a of cell 2 is thus aligned with the lower edge 8b of screen 8 and also with the point L attained by the maximum or longest light sheath around the electrode 6, whereas the lower edge 21) of the cell is aligned with the upper edge 8a of screen 8 and point L attained by the light sheath around the electrode 7. However, in this case the resistance of cell 2 is modulated, except for its inherent inertia, at a frequency which is twice that of the voltage applied to the lamp terminals.

On the other hand, the shape of the photosensitive surface of cell 2 may be properly selected to compensate at least partially the gain attenuation due to the gradually increasing distance between the upper portion of the light sheath and the lower portion of cell 2. Thus, the cell 2 may be constructed as shown in FIGURE 4 with a photoconductive surface 20 having a width increasing from top to bottom, so that the greater the distance between the luminous cathode sheath and the cell, the greater the transverse dimension of the illuminated photoconductive surface.

Under these conditions, it was observed experimentally that the ratio of the maximum and minimum resistances of the photoconductive cell 2 may be as high as 1,500 with a discharge current ratio of the order of 10.

The law governing the resistance variation may be modified by properly changing the shape of the electrodes as well as their relative gap according to operating requirements. In the cell illustrated in FIGURE 5 the electrode gap varies from one cell end to the other. In the form of embodiment illustrated in FIGURE 6 the photoconductive surface has a width variable in the longitudinal direction, with a threshold or level where this width changes to a substantial extent.

On the other hand it is clear that the form of embodiment of the present invention which is described hereinabove with reference to the accompanying drawing is given by way of illustration only and should not be construed as limiting the invention since many modifications may be brought thereto without departing from the spirit and scope of the invention.

Thus, notably, a discharge or glow lamp of a type other than the cylindrical bulb type may be used, provided that the cathode of the lamp is elongated sufficiently to permit the extension of the cathode luminous sheath.

The photorheostat according to this invention may also comprise a plurality of photocells distributed about the discharge lamp, these cells operating in parallel and being illuminated by the same variable light source. The photorheostat thus obtained is then of the type comprising an input circuit and a plurality of separate output circuits having either the same response curve (in the case of identical photoconductive cells) or different inherent response curves, according to the form of embodiment and the disposal of the photoelectric cells of the various output circuits.

Of course, the component elements of the photorheostat according to this invention may be selected to have shapes other than those described hereinabove, without inasmuch departing from the spirit and scope of the invention as set forth in the appended claims.

What I claim is:

1. A photorheostat comprising a variable light source consisting of a discharge lamp incorporating an anode and a cathode between which a variable voltage is applied, said cathode being parallel to the longitudinal axis of the lamp, a photoelectric cell disposed in front of said discharge lamp, and a screen disposed between said discharge lamp and said photoelectric cell, said screen extending along a certain length and overlapping the cathode end adjacent to the anode in order to prevent the direct transmission to said photoelectric cell of the light flux emitted from the end portion of the cathode which is adjacent to the anode, whereby, at the beginning of the discharge, the photoelectric cell is illuminated only by diffuse light.

2. A photorheostat as set forth in claim 1, wherein the cathode of said discharge lamp is filiform.

3. A photorheostat as set forth in claim 1, wherein said screen has a constant opacity.

4. A photorheostat as set forth in claim 1, wherein said screen has a variable opacity.

5. A photorheostat as set forth in claim 1, wherein said screen is inserted between the discharge lamp and the photoelectric cell along a certain length of the cathode of said discharge lamp.

6. A photorheostat as set forth in claim 1, wherein said screen consists of a wholly or partially light-tight substance consistent with the discharge lamp environment.

7. A photorheostat as set forth in claim 1, wherein said photoelectric cell is so disposed that it is completely illuminated only when the cathode light sheath has its maximum elongation around the cathode.

8. A photorheostat as set forth in claim 1, wherein the cathode and the anode of the discharge lamp are filiform and have substantially the same length.

9. A photorheostat as set forth in claim 1, wherein the cathode is longer than the anode.

10. A photorheostat as set forth in claim 1, wherein when said discharge lamp is fed with alternating electric current, the two electrodes of the discharge lamp are aligned on a common axis symmetrically in relation to a plane, the screen and the photoelectric cell being both symmetric in relation to said plane.

11. A photorheostat as set forth in claim 1, wherein said photoelectric cell is a photoconductive cell.

12. A photorheostat as set forth in claim 11, wherein said photoconductive cell comprises a photosensitive surface of which the width or transverse dimension varies as a function of the longitudinal direction in which the zone illuminated by the cathode light sheath develops when the discharge current increases.

13. A photorheostat as set forth in claim 11, wherein said photoconductive cell comprises electrodes of which the gap varies in the longitudinal direction in which the zone illuminated by the cathode light sheath develops.

14. A photorheostat as set forth in claim 1, wherein the light emission is rendered variable as a function of the parameters governing its intensity, notably by properly shaping the cathode of the discharge lamp.

References Cited UNITED STATES PATENTS 2,967,945 1/1961 DeGier. 2,997,630 8/1961 Kruse 250211 3,143,653 8/1964 Adams et al 250217 X 3,171,034 2/1965 Tomasulo et a1. 3,258,601 6/ 1966 Suleski. 3,363,106 1/1968 Park.

RALPH G. NILSON, Primary Examiner.

M. A. LEAVITT, Assistant Examiner. 

1. A PHOTORHEOSTAT COMPRISING A VARIABLE LIGHT SOURCE CONSISTING OF A DISCHARGE LAMP INCORPORATING AN ANODE AND A CATHODE BETWEEN WHICH A VARIABLE VOLTAGE IS APPLIED, SAID CATHODE BEING PARALLEL TO THE LONGITUDINAL AXIS OF THE LAMP, A PHOTOELECTRIC CELL DISPOSED IN FRONT OF SAID DISCHARGE LAMP, AND A SCREEN DISPOSED BETWEEN SAID DISCHARGE LAMP AND SAID PHOTOELECTRIC CELL, SAID SCREEN EXTENDING ALONG A CERTAIN LENGTH AND OVERLAPPING THE CATHODE END 